1. Field of the Invention
The present invention relates to a technique for controlling the phase of a pixel clock to correct the positional offset of dots in an imaging apparatus, and to a technique for detecting a scanning beam in synchronization with optical writing to control the phase of the pixel clock.
2. Description of the Related Art
FIGS. 1A and 1B schematically illustrate conventional imaging apparatuses. In FIG. 1A, a laser beam emitted from a semiconductor laser unit 1001 is reflected by a rotating polygonal mirror 1002 in a scanning direction, and focused by a scanning lens 1003 onto a photosensitive unit 1004. The focused laser beam forms a light spot and produces an electrostatic latent image on the photosensitive unit 1004. Laser driving circuit 1007 controls the light emission time of the semiconductor laser unit 1001, based on pixel data generated by an image processing unit 1006 and the pixel clock whose phases are determined by a phase synchronizing circuit 1009. The scanning laser beam is detected by a detector 1005, and the detection signal is supplied to the phase synchronizing circuit 1009.
In FIG. 1B, a pair of detectors 1010 are positioned in a plane extended from the surface of the photosensitive unit 1004 to detect synchronization of the scanning laser beam. Detection signals are supplied from the detectors 1010 to a counter 1011, and then to the look-up table 1012. The timings of the two detection signals are supplied to the phase synchronizing circuit 1009, which determines the phase of a pixel clock and supplies the phase-determined pixel clock to the image processing unit 1006. The laser driving circuit 1007 controls the light emission time of the semiconductor laser unit 1001, based on pixel data generated by an image processing unit 1006 and pixel clocks whose phases are determined by the phase synchronizing circuit 1009. In this manner, the position of the electrostatic latent image formed on the photosensitive drum (or the scanned medium) 1004 is regulated in the scanning direction.
In these imaging apparatuses, the position for forming an image has to be precisely regulated. Japanese Laid-open Patent Publication 2000-238319 discloses a technique for correcting the start position of writing an image of each color within an error range of one clock in a color laser printer. Another publication, Japanese Laid-open Patent Publication 2000-289251, discloses a technique for adjusting the start position and the end position of optical image writing in the main scanning direction.
However, even if controlling the start position and the end position of writing an optical image, the scanning rate of the light spot (formed by the scanning beam) moving on the photosensitive unit fluctuates in the conventional optical writing system. Such fluctuation of the scanning rate is due to variation in distance from the optical axis to the reflecting surface of the deflector (such as a polygonal mirror). If the scanning rate fluctuates, dot positions formed on a sheet are offset from the correct positions in the main scanning direction, which results in image fluctuation and deteriorated image quality.
When using a multi-beam optical system with multiple color light sources with different oscillation wavelengths, exposure positions of the respective color beams are offset from each other unless the chromatic aberration of the scanning lens is corrected accurately. In this case, the displacement of the light spots of the respective color beams differ from each other on the scanned medium (e.g., the photosensitive unit), which causes the image quality to deteriorate.
It is difficult to correct the displacement or positional offset of light spots or dots because the fluctuation of the scanning rate itself varies along the scanning line due to the characteristics of the optical systems used in the imaging apparatus.
Fluctuation of the scanning rate and positional offset of dots are likely to occur especially when the following factors arise:    1) the fθ characteristic of the scanning lens is not sufficiently corrected;    2) precision of the optical parts of the optical scanning system and assembling precision of the components onto the housing are insufficient;    3) the focal length changes due to deformation and change of the indexes of refraction of the optical components, which are caused by environmental changes, such as temperature change and humidity change, in the imaging apparatus;    4) the distance from the optical axis to the reflecting surface of the deflector (e.g., the polygonal mirror) varies, and the moving rate of the light spot on the scanned medium changes, as has been explained.
Neither prior art publication 2000-238319 nor 2000-289251 provide teaching that can sufficiently correct the adverse influence of positional offset of the dots occurring in the main scanning direction due to characteristics of optical systems or the deflectors.
Japanese Laid-open Patent Publication H6-59552 discloses a technique for correcting positional offset of dots in an optical scanner of a multi-point synchronizing type. This technique aims to correct displacement of the dots, which is caused by fluctuation of the rotational speed of the deflector or the variation in machining precision of optical system components. However, this technique is directed to changing the frequency of the PLL in order to control the dot position. With this method, the clock signal fluctuates due to the influence of the frequency change during the lock-up time of the PLL, and consequently, the phase of the pixel clock cannot be precisely regulated.
Another problem in the prior art is that a huge amount of correction data is required to make correction to the entirety of image data when correcting the fluctuation of the scanning rate. This problem causes the circuit scale to become large, and the cost for the control system also increases.
Still another problem is that if the phase of the pixel clock is corrected, without flexibility, based on the shift data, the corrected portions are repeated at the same positions in the sub-scanning direction, and vertical streaks appear in the resultant image.
There is yet another problem in the conventional optical writing system shown in FIG. 1B, which is separation of light flux. A portion of the light flux has to be guided to the synchronization detectors 1010 in order to control the dot position, while the remaining portion of the light flux is guided to the effective writing area on the scanned medium (photosensitive unit) 1004. To separate the light flux, the optical scanning system, including a deflector (e.g., a polygonal mirror) and other elements, inevitably becomes large. In addition, separating a portion of the light flux from the light beam guided to the effective writing area deteriorates the detecting precision. This problem becomes conspicuous especially in a multi-color imaging apparatus with multiple optical scanning systems.
It is important for the multi-color imaging apparatus to reduce relative inclination and bend of the scanning line, as well as to reduce the full-width or a partial magnification error in the scanning direction. The initial characteristics of the optical systems may be corrected by measuring the acquisitiveness prior to assembling the systems. However, errors that occur due to environmental changes as time passes, have to be measured and corrected during the operation of the apparatus.
In general, inclination of the scanning line and the full-width magnification error are measured on both sides of the effective writing area, while the bend of the scanning line and the partial magnification error have to be measured within the effective writing area. This separation of light flux between the effective writing area and the detecting positions on the both sides is difficult.
There is a known system in which multiple optical scanning systems are arranged in the main scanning direction to scan the beams on the non-scanning plane. However, it is more difficult for such a system to extract a portion of light flux in order to detect synchronization because of the layout design of arranging the optical scanning systems in the main scanning direction.